SPIGOT-1677: Fix attributes with only single underscore.

src/main/java/org/bukkit/craftbukkit/attribute/CraftAttributeMap.java

@@ -29,8 +29,10 @@ public class CraftAttributeMap implements Attributable {
         StringBuilder sb = new StringBuilder(bukkit.toLowerCase());
 
         sb.setCharAt(first, '.');
+        if (second != -1) {
             sb.deleteCharAt(second);
             sb.setCharAt(second, bukkit.charAt(second + 1));
+        }
 
         return sb.toString();
     }